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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Asset Retirement Obligations [Line Items]    
Balance, beginning of the period $ 2,497 $ 1,838
Accretion 52 167
Changes in cash flow estimates 417 492
Balance, end of the period $ 2,966 $ 2,497